The Collaborative International Dictionary of English v.0.48:

flavorless \fla"vor*less\, a.
   Without flavor; tasteless. Opposite of flavorful.

   Syn: bland, flat, flavourless, insipid, savorless.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

flavorless
    adj 1: lacking taste or flavor or tang; "a bland diet"; "insipid
           hospital food"; "flavorless supermarket tomatoes"; "vapid
           beer"; "vapid tea" [syn: bland, flat, flavorless,
           flavourless, insipid, savorless, savourless,
           vapid]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "flavorless":
   dead, dilute, diluted, distasteful, fade, flat, gruelly,
   ill-flavored, inane, indifferent, insipid, jejune, mild,
   milk-and-water, pappy, pulpy, sapless, savorless, spiceless, stale,
   tasteless, thin, unappetizing, unflavored, unsavory, vapid, washy,
   watered, watered-down, watery, weak, wishy-washy